This paper describes an experiment to investigate the best way for lay people to use representation to visualise and negotiate space. It was motivated by our observations in workshops for digital inclusion in the context of a housing project for a homeless association. Computers were used to make it easier for the community to understand and change the spaces in real time. The first workshops proved that our approach was efficient as an exercise but not certainly effective concerning the understanding of spatial qualities. So we have designed an experiment to compare the usability of different media in participatory design processes. For that we have adapted the ‘Usability’ methodology, which is fully described in the paper. We started with three main questions. The first concerned the effectiveness of different media to represent spatial quality; the second concerned the best way for novices to approach space, whether by refurbishing a pre-existing space or by starting from the scratch; and the third concerned the effectiveness of negotiation by means of discourse and by means of or action. We also had two main hypothesis: one coming from research on digital environments and stereo visualisation, indicating that the more people feel immersed in the represented environment the more they are able to correlate it with physical space; and the other coming from our own observations in the participatory design workshops, in which the collective decision-making was manipulated by those people with more advanced communication skills who use their ability in an authoritative way regardless of the relevance of what they have to say. This paper describes the whole experiment, which was an exercise of spatial negotiation in 5 versions. In the first version we provided fixed digital views of a room in plan and axonometry; for another two versions we provided a physical model of the room in 1:10 scale, with some pieces of the existing furniture in different scales. This was done to check if people were just playing with a puzzle or actually grasping the correspondence between representation and the object or the space represented. One version proposes refurbishment and the other starts from the scratch. And the last two versions repeated the same task made with the physical model, but this time using a 3D interactive digital model. People were required not only to organise the furniture in the space but also to build a full scale cardboard structure and organise the real furniture reproducing their proposed model. Their comments on the spaces they had built confronted with what they had imaged when working with the model has enabled us to compare the different models, as also the different ways of negotiating spaces. This paper describes this experiment in detail concluding that 3D digital interactive models are far more effective than physical models and 2D drawings; when negotiation happens by means of action it provides more creative results than when the discoursive practice prevails; people are more creative when they start something from scratch, though they spend more time. The results of this experiment led us to formulate a new hypothesis leading to the development of an immersive collaborative environment using stereoscopy.